1. Field of the Invention
The present invention relates to an optical recording medium including a plurality of recording and reading layers, and an optical recording-reading method of recording information on the optical recording medium.
2. Description of the Related Art
Conventionally, optical recording media such as CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD+/−RWs, DVD-RAMs, Blu-ray Discs (BD) are widely used to view digital video contents and to record digital data. In the BD standard, which is one of the next generation DVD standards, the wavelength of the laser beam used for recording and reading is reduced to 405 nm, and the numerical aperture of an objective lens is set to 0.85. In optical recording media conforming to the BD standard, tracks are formed at 0.32 μm pitch. This enables 25 GB or more data to be recorded on and read from one recording and reading layer of such an optical recording medium.
It is expected that the size of video and data files will increase more and more in the future. Therefore, it is contemplated to increase the capacities of optical recording media by using a multiple stack of recording and reading layers. In the technologies for BD standard optical recording media reported by I. Ichimura et al., Appl. Opt., 45, 1974-1803 (2006) and K. Mishima et al., Proc. of SPIE, 6282, 628201 (2006), 6 to 8 recording and reading layers are provided to achieve ultra large capacities as much as 200 GB.
When a multiple stack of recording and reading layers is used in an optical recording medium, projections and depressions, such as grooves and lands, for tracking control are formed on each recording and reading layer, and therefore a stamper (a master die) must be used to form the projections and depressions each time a recording and reading layer is formed. Therefore, as the number of stacked layers increases, the number of times the stamper is used increases, and this may result in an increase in manufacturing cost.
In the technologies for optical recording media recently proposed in Japanese Patent Application Laid-Open Nos. 2008-97693, 2008-97694, and 2008-108383, servo layers are provided separately from recording and reading layers, and information is recorded on each recording and reading layer using a recording laser beam while a tracking signal is obtained from a corresponding one of the servo layers using a servo laser beam. With such technologies, the recording and reading layers need not have projections and depressions (grooves) for tracking information, and a stamper need not be used for the recording and reading layers during manufacturing. This enables a significant reduction in cost. It is also proposed that a plurality of servo layers are used to facilitate aberration correction and to allow two layers to be read simultaneously.
In the optical recording media described in Japanese Patent Application Laid-Open Nos. 2008-97693 and 2008-97694, the amount of spherical aberration correction can be reduced, and therefore recording-reading accuracy can be improved. However, the problems with such optical recording media are that it is difficult to record and read information on/from the media efficiently. More specifically, one of the problems is that, when information is recorded on the recording and reading layers sequentially in their stacking order, the seek time required to change from one recording and reading layer to the next is long. Another problem is that, when two servo layers are used alternately, the beams must be moved considerable distances in a focusing direction between the servo layers and between the recording and reading layers, so that the moving time and adjustment time increase. Yet another problem is that, when the beam for the servo layers and the beam for the recording and reading layers have the same wavelength, the reflected light of the beam for the servo layers may adversely affect recording on and reading from the recording and reading layers and that the reflected light of the beam for the recording and reading layers may adversely affect the tracking control performed using the servo layers. Therefore, mutual interference must be reduced by changing materials and structure design.
The positions of the recording and reading layers of such an optical recording medium in its thickness direction are unknown on the side of an optical pickup that records and reads information on/from the optical recording medium. There are various optical recording media that are different in the number of recording and reading layers. Therefore, each time an optical recording medium is inserted, the point on which the recording laser beam is focused must be moved in the focusing direction to read the position of each recording and reading layer. This results in a problem in that the preparation of recording and reading is time consuming.